1. Field of the Disclosure
The present disclosure generally relates to cement pulsation for a subsea wellbore.
2. Description of the Related Art
A wellbore is formed to access hydrocarbon bearing formations, such as crude oil and/or natural gas, by the use of drilling. Drilling is accomplished by utilizing a drill bit that is mounted on the end of a tubular string, such as a drill string. To drill within the wellbore to a predetermined depth, the drill string is often rotated by a top drive or rotary table on a surface platform or rig, and/or by a downhole motor mounted towards the lower end of the drill string. After drilling to a predetermined depth, the drill string and drill bit are removed and a section of casing is lowered into the wellbore. An annulus is thus formed between the string of casing and the formation. The casing string is cemented into the wellbore by circulating cement into the annulus defined between the outer wall of the casing and the borehole. The combination of cement and casing strengthens the wellbore and facilitates the isolation of certain areas of the formation behind the casing for the production of hydrocarbons.
It is common to employ more than one string of casing or liner in a wellbore. In this respect, the well is drilled to a first designated depth with a drill bit on a drill string. The drill string is removed. A first string of casing is then run into the wellbore and set in the drilled out portion of the wellbore, and cement is circulated into the annulus behind the casing string. Next, the well is drilled to a second designated depth, and a second string of casing or liner, is run into the drilled out portion of the wellbore. If the second string is a liner string, the liner is set at a depth such that the upper portion of the second string of casing overlaps the lower portion of the first string of casing. The liner string may then be hung off of the existing casing. The second casing or liner string is then cemented. This process is typically repeated with additional casing or liner strings until the well has been drilled to total depth. In this manner, wells are typically formed with two or more strings of casing/liner of an ever-decreasing diameter.
The migration of gas from a hydrocarbon bearing formation into the cement slurry may occur after the cement has been pumped, but before it has fully cured. The consequences include gas cut cement, sustained casing pressure, and/or blow outs to the surface. The control of gas migration is one of the most costly and challenging technical problems in well cementing. The basic cause of gas migration is believed to be the loss of hydrostatic pressure within the cement column as it makes the transformation from a liquid slurry to a solid. The development of gel strength in the static column of the curing cement slurry is primarily responsible for this loss of hydrostatic pressure. This loss of hydrostatic pressure allows an influx of gas before the cement slurry has completed the curing process.
Gas migration can be prevented if gelling of the cement slurry can be prevented or delayed until the cement slurry develops enough viscosity to prevent the movement of gas within the slurry. Gelling can be disrupted by mechanical agitation, such as by rotation of the casing or liner string. However, rotation must be stopped when the drag on the casing or liner string at the bottom of the well becomes too high and before torque builds to the point that the casing or liner string might be twisted off. This may occur before the cement slurry is viscous enough to prevent gas migration at shallower depths because the cement slurry tends to cure faster at the bottom of the wellbore due to the higher temperature. Gas pulsation has also been used to disrupt gelling in subterranean and shallow water wells having surface wellheads but is unsuitable for deeper wells having subsea wellheads due to the risk of riser collapse and/or buoyancy destabilization of the floating offshore drilling unit.